Manufacture of wet process phosphoric acid



Sept. 24, 1963 D. J. VEN. 3,104,946

MANUFACTURE 0F WETV PROCESS PHOSPHORIC ACID Filed Dec. 28, 1959 5 Sheets-Sheet 1 Vl wm .m v r nl@ 1|. om Ekm 0H... am?? ...ms 5.5.... Gom mrnorn.

Sept. 24, 1963 D. .1.A VEAL MANUFACTURE oF WET PROCESS PHosPHoRIc ACID Filed Dec. 2s, v1959 5v Sheets-Sheet 2 .mm om n .Erd

A TTOPNEVS Sept. 24, 1963 D. J. vEAl.

MANUFACTURE OF WET PROCESS PHOSPHORIC ACID 3 Sheets-Sheet 5 Filed DeC. 28, 1959 INVENTOR.

D J VEAL MJA i" A TTORNEVS United States Patent 3,i04,946 MANUFACTURE 0F WET PROCESS PHOSPHRIC ACID Dean J. Veal, Bartlesville, kla., assigner to Phillips Petroleum Company, a corporation of Delaware Filed Dec. 28, 1959, Ser. No. 862,136 Claims. (Si. 23-1-55) This invention relates to the manufacture of wet process phosphoric acid. More particularly, it relates. to a process and apparatus for automatically controlling the amount of free' sulfuric acid in the production of phosphoric acid according to the Wet process.

Many processes have been suggested, patented, or employed in the past for the manufacture of phosphoric acid. The so-called wet process for producing phosphoric acid has been extensively used because of its simplicity and economical operation. According to this process, phosphate rock is treated in a series of reactors with concentrated sulfuric acid to produceV a slurry of dilute, soluble phosphoric acid and insoluble calcium-sulfate (gypsum). 'Ille slurry is filtered and washed and the resultingdilute phosphoric acid filtrate product may then be concentrated for the production of triple superphosphate or other phosphatic fertilizer material. The insoluble calcium sulphate is usually reslurried with water and pumped to a spoil pond for disposal. It has recently been discovered that it is advantageous to initially react the phosphate rock with recycle weak phosphoric acid filtrate.

One of the most important variables which affects the reaction rate of the rock with lthe acid, the lterability of the reaction slurry, and the grade and physicalproperties of the phosphoric acid product, is the free sulfuric acid content in the wet process system. The percent Ifree sulfuric acid is the percent of excess sulfuric acid above that required to react with all the phosphate rock. Although Vother process variables, such as rock particle size, reaction time, and temperature, are important, these can be fixed by plant design and experience. But it is the free sulfuric acid content in the system (which determines the acid-torock ratio) that is probably the most sensitive variable in the production of phosphoric acid by the wet process. It too little free sulfuric acid is present in the system, the lterability of the reaction slurry is aected because the reactors tend to produce a non-uniform lter' feed slurry which is diicult to filter and which forms a filter cake that is difficult to wash to a low,Water-soluble P205 content. A low sulfuric acid content causes the formation of very slimy, jelly-like impurities like silicic acid which get into the gypsum filter cake causing the same to blind and hinder effective filtration of the acid through the cake. Such a filter cake retains a small but economically significant amount of P205V and when the gypsumv filter cake is discarded valuable amounts of available P205 are lost, thus affecting the economical operation of the process.

lf the free sulfuric acid content in the system becomes too high, the amount of sulfuric acid in the filtrate product is correspondingly high and when this product is concentrated and used for reaction With phosphate rock in the production of triple superphosphate, the free sulfuric acid reacts with the rock to form calcium sulfate instead of triple superphosphate. This makes a low grade of triple superphosphate because the available P205 content of the triple superphosphate is lowered. Also, this high sulfuric acid content causes the triple superphosphate to become sticky and harder tohandle. Another objection to the high sulfuric acid content in the system is that it increases corrosion in the system. Further, if the free sulfuric acid is allowed to become too high, the solubility of calcium sulfate is decreased to a point where ICC the sulfuric acid will react with the larger particles of the phosphate rock feed before they are dissolved in the phosphoric acid. These particles become coated with calcium phosphate and this stops .part of the rock from dissolving and reacting with the sulfuric acid. The reaction then stops using a large portion of the sulfuric acid that is being added and the free sulfuric acid becomes very high in the short period of time. 0f course, the use of too much sulfuric acid in the process is wasteful in itself.

Accordingly, an object of this invention is to improve the manufacture of wet process phosphoric acid. Another object is to provide improved apparatus and process-for automatically controlling the amount of free sulfuric acid in the production of phosphoric acid according to the wet process. Another object is to improve the reaction between phosphate rock and concentrated sulfuric acid in the production of phosphoric acid. Another object is to improve the lilterability of the reaction slurry obtained by acidulating phosphate rock with concentrated sulfuric acid. Another object is to improve the recovery of available P205 in the production of phosphoric acid according to the wet process. Another object is to provide lfor the economical and efficient use of sulfuric acid inthe production of phosphoric acid according to the Wet process. Further objects and advantages of this invention will become apparent to those skilled in the artfrom the following discussion, appended claims, and accompanying drawing wherein:

FIGURE 1 diagrammatically represents a system for treating raw materials to produce the desired product according to the present invention;

FIGURE 2 diagrammatically represents a ydetailed portion of the analyzing system shown-in FIGURE 1; andl FIGURE 3 schematically illustrates an electrical system which can be used in the automatic operation of this invention.

By convention, the phosphoric content of any type of phosphatic fertilizer is expressed as phosphoric anhydride (P205). Most commonly, the term phosphoric acidr is used to denote the P205 content; The difference between the total P205 content of the fertilizer andthe fraction that is insoluble in ammonium citrate solution is the portion caused to be available for use of the plant and hence is termed available P205, or available phosphoric acid. In thefertilizer trade, it has long been the custom to express the quantity of phosphate rock in terms of P205 since tricalcium phosphate is seldom if ever found in phosphate rocks as such.

Referring now tothe drawing, and to FIGURE 1 in particular, phosphate rock raw material is conveyed via line 1 to rock storage and grinding facilities'Z'. From the latter, nely divided phosphate rock is transferred by conveyer belt 3 to a line rock feed bin 4 provided'witha suitableV rock feeder 5 at the low end thereof, such as a star valve. The line rock drops onto a weigh belt 6- and is accurately weighed and conveyed' at a constant rate to aV miying device 7, such as a tank or trough, where it is initially acidulated with Weak phosphoric acid recycle conveyed via line 8. Mixing tank 7A is providedI with a suitable motor-driven agitatorV so as to insure complete wetting of the rock with the recycle acid. This recycle acid, preferably having a specic gravity of about 1.2, serves toV initially react with the rock, to furnish the water to the' process necessary to dilute the phosphoric acid formed in reactors to a specific gravity of about 1.3, to form a gypsumV slurry, and to provide Water vfor evaporation toV coolV the reactors. For a given rock rate, the recycle rater of this acid is held constant; this rate controls theA amount of solids in the last reactor, in which the solids content is preferably' held at about 25 percent.

The wetted rock is allowed to iiow after a few seconds into the lirst of a series of four reaction tanks or reactors 9, 10, 11, and 12, which are provided with suitable agitation means and are preferably enclosed and fabricated from carbon steel lined with rubber and acid-proof brick.

The wetted rock is mixed and partially reacted with concentrated sulfuric acid in the first reactor 9, the acid being supplied from storage tank 13 via line 14. The resulting mixture then overflows into the second reactor Where additional sulfuric acid is preferably added via line 16. The rock slurry mixture then overflows into third reactor 11 and then from there into the last reactor 12. Additional sulfuric acid can be added to reactors 11, 12, if necessary. The reactors are in series to provide sufficient retention time (eg, 2 hours) for the reaction to go to completion and form phosphoric acid and gypsum. Obviously, l do not intend to limit my invention to the use of four reactors but instead to broadly include the use of any one or more, though I prefer the four as shown.

The reaction between sulfuric acid and phosphate rock is exothermic and considerable quantities of heat are liberated by this reaction, as Well as from the dilution of concentrated sulfuric acid, the latter being initially diluted bythe recycle phosphoric acid, for example, to a concentration of about 93 percent. The reactors will generally be operated at a temperature in the range of 165 to 190 F., but for the production of phosphoric acid having a maximum P205 content of 28 percent, a temperature of 170 to 175 F. is preferred. The fumes and steam evolved from the reaction can be carried by suitable means to fume scrubbers, and the like. The slurry of soluble phosphoric acid and insoluble calcitun sulphate (gypsum) is discharged from the last reactor 12 via filter line 17. About twothirds of the phosphoric acid, P205, in the last reactor 12 comes from the recycle acid and about one-third is produced in the reaction of the phosphate rock. There are also various impurities dissolved in the phosphoric acid, such as the phosphates of iron and aluminum, various fluorides and silica compounds. The filter feed is discharged from line 17 onto one or more filters 18. ln the interest of simplicity and to avoid prolixity, only one filter is shown and is preferably a horizontal vacuum filter of the Oliver type. it is to be understood and I do not intend to limi-t the practice of my invention to any particular type of vacuum filter. Although l prefer to employ a plurality of vacuum filters, c g., three, actually one lter could be used if the filtrate pumps are large enough to handle all of the filtrate from the slurry.

The filter feed is fed over the surface of a lter cloth on what is called the strong filtrate section of the filter. Here `as much of the undiluted phosphoric acid as possible is pulled from the deposited gypsum filter cake by a vacuum. The filtrate obtained, known as the strong acid filtrate, or acid product, passes through the filter cloth and flows into a filtrate receiver 19 via line 21. This filtrate, a product of the process, preferably has a specific gravity of about 1.30. Generally, only a major portion of the strong acid, eg., 70 percent, will be pulled through the filter cake, the minor portion remaining in the filtercake as residual strong acid. During .the next filtration step the residual strong acid is recovered by Washing the filter cake with Weak wash filtrate.supplied via line 22. The filtrate thus obtained, known as the first Weak acid filtrate is drained through` line 23 to a second filtrate receiver 24. By this second filtration step, most of the residual strong acid is recovered; however, usually about one-third of the residual strong acid will still be retained by the filter cake and must be recovered. Accordingly, the filter cake is then washed with relatively hot water, e.g.,.l70 F., to displace any residual acid remaining in the cake Vafter'the first Washing step; filter wash -water is conveyed via line 26 from supply Z7.V The filtrate obtained, known as the vsecond weak acid filtrate, is drained into the third filtrate receiver 23 via line 29. A portion or all of this last filtrate 4 is pumped back through line 22 for use in the first washing as described above. The specific gravity of this filtrate is preferably in the range of 1.05 to 1.08. After the water lWashing step, the filter cake is removed with a blade that cuts off the top portion of the filter cake, which is then sent to a spoil pond via line 31.

The strong acid product in receiver I19 is pumped via line 32, 33 to suitable phosphoric acid concentration means 34 where'the strong acid filtrate product can be concentrated, for example to 49.2 percent P205. The first weak acid filtrate in receiver 24.1 is pumped via lines 36, 37 to a recycle storage or surge tank 33 from which the acid is recycled via line S to the mixing tank 7. When necessary, some of the first weak acid filtrate can be conveyed from line 36 to line 33 by opening valve 39, and some of the second weak acid filtrate can be conveyed from the filtrate receiver 2d via line 41 to the weak acid recycle line 8 by opening valve 42.

As explained hereinbefore, it is desired to automatically control the amount of free sulfuric acid in the system according to this invention. Broadly, this is accomplished by controlling the acid-to-rock ratio by automatically adding the concentrated sulfuric acid `after correcting for the amount of free sulfuric acid in the recycled phosphoric acid stream.

This is accomplished according to one preferred embodiment of this invention in the following manner. Referring to FIGURE 1, the weight of rock added to mixing tank 7 is determined by the product of signals representing weight per vunit length of the weigh belt 6 and the speed of the Weigh belt. `A section of the Weigh belt v6 is suspended so that equal lengths of the loaded belt and return belt are balanced on opposite sides of a fulcrum 44. The position of the fulcrum 4d determines the voltage drop across a strain lgage in a weighing device 47. The signal from the latter is conveyed to a feed rate instrument 4S, such as a multiplying circuit or totalizer. The speed of weigh belt 6 is determined by belt speed indicator 49 and this alsofsends a signal to the feed rate instrument d8. With no load on the weight belt 6 the system is in balance. As the load on the Weigh belt 6 approaches the desired rate, a signal representing weight per unit length of the belt is conveyed to the feed rate instrument 48 along with a signal proportional to the belt speed. VThese signals are multiplied in feed rate instrument 3S and the product propontiona-l to the rock feed rate is sent to a computer controller 5f) where it will be electrically multiplied by a signal representing the free sulfuric acid content of the system. This computer controller 5d is connected to a fiow recorder controller S1 which in turn is operatively connected to a flow control valve 52 in the concentrated sulfuric acid line 14, controllingl the operation thereof. The computer controller Sil is also operatively connected to a fiow recorder controller 53 which in turn operatively is connected-to a flow control valve 54 in the filter Wash Water line 26, controlling the operation thereof. The computer controller 50 is also operatively connected to an analyzer 55 which is operatively connected by a small sample line 56 to the filter feed line 17. Alternatively, the analyzer 55 can be operatively connected to analyze the free sulfuric acid content in recycle line 8, or product line 32,

or any other convenient point in the system. Analyzer 55 is adapted to continuously analyze the free sulfuric acid content, expressed as percent S03 in the system. The computer controller Stils operatively adapted to control the amount of concentrated sulfuric acid feed conveyed in line 14 and the amount of filter wash water conveyed in line 2,6 in such a manner as to maintain the free acid content in the system within predetermined limits.

In the practice Vof this invention, it is preferred thatV the S03 content of the slurry be maintained within the range or" about 0.5 to 3.5 percent S03, preferably within the range of about 2.0 and 2.5 percent S03. Porexample,

of sulfuric acid being continuously added within the predetermined limits.

Should the analysis of the slurry show that the percent S03 is above the broader desired range, e.g., greater vthan 3.5 percent S03, the computer controller 50 accordingly sends a signal to the filter wash water stream control valve '54 and correspondingly increases the liow of wash Water sent to the filter 18, thus causing dilution of the second Weak acid filtrate conveyed from recycle tank 38. At the sameY time, the computer cont-roller Stlsendsa larger signal to the ilow control valveSZ in the sulfuric acid feed line 14 so as to further decrease the amount of sulfuric acid sent to the reactors by a suitable increment, eg., 0.4 ton/hr. When the S03 content of the slurry returns to the narrower desiredV range, the computer con- ,troller 50 accordingly controls the concentrated acid flow control valve 52 to restore the normal addition rates. As the S03 content of the slurry falls, the control valve 54 on theV filter washwater line26 will be reSAetlin increments, eg., 5 tons/hr., until the normal Wash rate on the filter 13 has been restored.

lf a severe upset occurs resulting inra low S03 content in the slurry, for example, below about 0.5 weight percent, which might be caused by the rock having a high P205 content, the rock addition system operating on the high side, 'and the high :filter Wash rate, all occurring simultaneously, control of the free sulfuric acid -is again dictated; This condition should be ladiusted by the proper addition of makeV up acid as controlled by computer controller 50, valve 52 being opened to the desiredextent. A suitable siren can be actuated by controller 50-when this condition occurs to make the operator aware of the low S03; The operator can then adjust some of the difficulty manually by diverting the first-Weak acid filtrate in line 36 to the concentratorl 34 by opening valve 39. Alternativley, this diversion can be also automatically controlled by operatively connecting valve 3-9 :to controller S0.

Although any suitable analyzer can be used-forv analyzing .the percent S03 content of the'slurry, theV analyzing system illustrated in FIGURE 2 is a preferred'embodiment.

The analyzing system of FIGURE 2 automatically measures the amount of unreacted sulfuric acidlpresent in the filter slurry feed. At a :1 ratio of phosphoric acid to sulfuric acid, the difference in the second ionization constants of the two acids is great enough to permit differential titration for the determination ofA both acids. Titration -to a pH of 4.8 measures the sulfuric acid content plus one-third of the phosphoric acid content; Titration to a pH of 9.0 measures the sulfuric acid content plus two-thirds of the phosphoric acid content. Subtraction of one-half of the second titration from the first gives amount of sulfuric acid .present in the mixture. The sample stream 56 of the slurryV feed 17 is diluted and then neutralized with a standard base, sodium hydroxide, and the neutralizedr stream Ihas its two pHs-measur'ed by a pair of antimony or glass electrodes located downstream from the standard base inlets by means oftwo'A^.C.-oper ated pH meters. Y

Referring now Ito FIGURE 2 in detail, a sampley stream of the slurry liowing in line 17 is diverted by wayA of line 'S5 to a suitable bellows pump 61 having a variable speed motor 62 and a pump speed controller 63f connected thereto. Deionized'water is pumped via line p'bymeans of pump 67 having a variable speed motor 68and'pump speedY controller 69 connected thereto.` This deionized water is pumped via line 65 to a vesselV 71 where it is mixed with 'the sample slurry stream entering by way of line 561 A ratio controller 70 actuated by pump speed controller 63 and pump speed controller y69 maintains a ratio of 99 to l between the flow ofdeionized Water in line 65 to the flow of slurry in line 56 respectively. Mixture in vessel 71 passes by way of line 72 and la control valve to a second mixing Vessel 74. Sodium hydroxide having a normality of about 0:1 equi-valent per liter is introduced to mixing vessel 74 by means of pump 76 through line 77. Pump 76 is equipped with a variable speed motor 7,3 and pump speed controller 79. The mixturel from vessel 74 flows byV way of line 80 into a third vessel 82. Mixture in line y8l) contacts electrode 83, setting up a pH cell, the pH of which is determined by pH meter84. Signals from pH meter 84 controlV the speed of motor 73 yand thus the rate ofV addition of sodium hydroxide to vessel 74. This system is adjusted to maintain the pH at the electrode 83 `at 4.8i0-05. Thepump speed controller 79 also transmits a signal to an electrical subtracting circuit 85. The mixture in vessel 82. is mixed with additional sodium hydroxide introduced via line S6. The mixture from thisvessell 82 flows via line `37 'to waste; it also passes through a pH cell containing electrodes 88 Whose signal operatespH meter 89 Which transmits an electrical signal to pump cor'1troll'er'90V on motor 91 thereby controlling the rate of 110W of sodium hydroxide to vessel' 32.V This system isV adjusted' so that the pH of the material passing through the secondpH cell has a value of 9L0i0-05'. The signa'l'from the pump speed controller 90 is also sent to subtracting circuit 85.

Referring now to the electrical diagram of FIGURE 3, an automatic control'A s'ysternwhich can be used'in this invention is illustrated. The voltage drop acrossb'ridge 161" of Weighing device 47 is amplified' by amplifier 102 and the output therefrom applied acros's potentiometer 103. The contactor 104of potentiometer 106 is moved by the mechanical linkage 107 of belt speed indicator \49j, an dthe'potentialpickedoffbycontactor 104 is ampliiied by ampliier 108. The y.amplified signal from amplilierV 108v -drives servo motor 109i' which causes mechanical linkage 111 to move contactor 112 of potentiometer 103. This results in thel multiplication of the weight of rock per unit length-of beltl6V times-thevspeed-of the belt in feet per unit le'rfrgthVV of time; the product being the feed rate of the rock per unit-length-ofY time;v The feed rate signalis amplified by amplier 113 and the amplified signal applied to potentiometer 114, the contactor 116 of which is moved by mechanical linkage 117 driven by servo motor 118, which in turn is driven inproportion lto the determined `free sulfuric acid content of the slurry.

The latter determination is made by electrically Vdividing the signal from pump speed controller gli by two-and subtracting` the quotient from the signal from pump speed controller 79. This is shown in FIGURE 3, where the signal from pump speed v controller 90 is divided by two by potentiometer 119. The reduced signal is subtracted from the sign-a1 from pump speed controller 79 in a subtracting circuit`121 having an amplifier 122.A The amplified signal is amplified by servo amplifier 123, the servo motor 11S movingmechanical linkage 117, as described above. i A' The product of the rock feed rate and free sulfuric acid content is amplified-by amplifier 124 and fed to servo amplifier 126 of ow recorder controller `53 and to servoa-mplier 127 yof ow recorder controller 51: r[The amplifiedsignal' from amplifier 127' drives the servo motor 128 in iiow recorder controller 51. The contactor 12'9" is driven -by servo motor 12'8'across slide wire 1?;1 of controller 51. When contactor" V129Y rrrovestothe extreme left, it closes switch 132,` lthe position ofv contactor '1291on slide WireV131beingrepresentative of a' free acid content of 2.6%; when switch 132 is closed, closed, the acid flow 'control valve 52' is closed' down to a4 lviipre-selfValue d' W'slV Water llW'contiol valve 7 54 is reset to its normal position, due to preset limit stops in controller 53. When contactor 129 moves to the eX- treme right, it closes switch 133, the position of contactor 129 on the slide wire 131 being representative of a free acid content of 3.6%; when switch 133 is closed, the acid flow control valve 52 is closed down further and wash water iiow control valve 54 is opened further to a position greater than its normal position.

The electrical system described above and illustrated in the 'drawing has been briefly set forth in the interest of brevity, the various potentiometers, ampliers, multiplying and subtracting circuits, iiow recorder controllers, and other components being well known in the art and commercially available.

To -further illustrate the object and advantages of this invention, reference will now be'made to the following example. However, it should be understood that the various rates, measurements, and other data in this example are merely representative of a preferred embodiment of this invention and should not be construed so as to unduly limit the invention.

' EXAMPLE Finely ground (7.0-80 percent through 20D-mesh screen) phosphate rock, having the analyses set forth in Table I, is transferred at a rate of 17.6 tons/hr. (6.36 tons/hr. of P205 content) from the grinding and storage facilities 2 by Way yof belt 3 to the iine rock feed bin 4. The ne rock is conveyed by weigh belt 6= to the mixing tank 7 at a desired rate of 19.5 tons/hr. The feed rate instrument 48 determines the feed rate of the tine rock within the limits of il percent or about i400 lbs. of fine rock; this is equivalent to about 130 lb./hr. of P205 content in the rock and would require about 337 lb./hr. of sulfuric acid for conversion. The signal from the lfeed rate instrument 48, which is proportional to the rock feed rate, is sent to the computer controller 50.

T able I TYPICAL ANALYSIS F PHOSPHATE ROCK Ingredient Weight '1X/Hr.

' Percent P205 32 .5e e .as

Y F 3 .59 0 .70 Combined water- 57 .49 11.20 Free water 2.87 0 .56 Non-reactive 8 .49 0 .68

Total 100 .00 19 .50

Recycled phosphoric acid containing approximately 17.04 tons/hr. of P20,-7 is pumped at a rate of approximately 260 gal/min. (81.56 tons/hr.) through line 8 to the mixing tank 7. The capacity of the mixing tank 7 is 1700 gals. and with a flow rate of approximately 101 tons/ hr., Ithe retention Itime in the mixing tank is approximately 5 to 6 minutes. On the basis of 81.56 tons/hr. of recycle phosphoric acid, 0.5 percent S03 conten-t represents about 800 lbs. of SO2 and 0.25 percent or one-half the desired range of 2.0 to 2.5 percent S03 would be equivalent to about 400 lb. S03, which will be noted to be 4about the sameorder of magnitude as the control of the rock. A typical analysis of the recycle phosphoric acid is set forth in Table II;

Table Il The ywetted rock overflows from mixing tank 7 to the series of reactors 9, 10, 11, and 12. 'I'he capacity of each of these reactors is 9790 gals. which fixes the total reaction time within the range of 2 hours to about 2 hours, 20 minutes. This is approximately 30 to 35 minutes in each reactor.

Table IH is a material balance for a plant when operating under the following conditions: ne rock 70-80 percent through 20D-mesh screen, reaction time approximately 2 hours, temperature l60-l90 IF., mostly within the range 17o-180 F., and 2.0 to 2.5 weiglht percent S03 content in the recycleacid.

Referring -to Table II, it will be noted that the S03 is 2.61 weight percent. When such an analysis is obtained and the S03 content is between 2.50 and 2.75, the sulfuric acid rate will be diminished by a signal from the come: puter controller 5,9 by an Vincrement of 0.2 ton/hr. (400 lbs/hr.) until an analysis between 2.0 and 2.5 percent S03 is again obtained, whence the rate will be adjusted again to the normal rate of 16.4 tons/hr. of sulfuric acid. Should the analysis of the slurry show that more than 3.5 iWeight percent S03 is present, the computer controller 50 laccordingly sends a signal t0 the r'ilter wash water valve 54 which results in an increase in the ilow of wash water from approximately 29 `tons/hr. :to approximately 43 tons/ hr. and approximately 13.5 tons/hr. of recycle lter Wash water from line 26 will be mixed with the recycle acid ilowing through line 8. This will immediately adjust the S03 content to the correct range, and prevent deposit of gypsum on the tine rock surfaces. At the same time, the sulfuric acid feed rate is decreased by 0.4 ton/ hr. When the S03 content returns to the range of 2.0 to 2.5 weight percent, the sulfuric acid feed will be raised in two increments of 0.2 ton/hr. -to restore the normal addition rate. As the SC3 content of the slurry ffalls, the lter wash water control valve 54 will be reset in increments of approximately 5 tons/hr. until the normal Water rate on the tlter 18 has been restored.

Various modicatious and alterations of this invention .will become apparent to those skilled in the art from the foregoing 'discussionand accompanying drawing, and it should be understood that the subject invention is not to be unduly limited thereto.

I claim:

1. In the process for the production of phosphoric acid laccording to the Wet process wherein a feed of phosphatic material and sulfuric acid are controlled, said process including the steps of initially reacting phosphatic material |with recycled Weak phosphoric acid, reacting the resulting mass with concentrated sulfuric-acid to obtain a slurry of phosphoric yacid and gypsum, ltering said slurry in a filtration zone to `obtain a strong acid ltrate comprising strong phosphoric acid and a remaining mass comprising gypsum, was-hing the remaining gypsum mass with a Wash medium to yobtain a first Weak ltr-ate, Washing the now :remaining mass with Water to obtain a second weak acid iiltrate, using said second weak acid filtrate as saidwash medium, yand recycling weak Ifiltrate for use as said recycled weak phosphoric acid, the improvement comprising analyzing the free sulfuric acid `content in a stream resulting in the process and containing free sulfuric acid by diluting a portion of said stream with water, subsequently 9 neutralizing the resulting diluted portion with standard sodium hydroxide base to fa pH of about 4.8 and then to a pH of about 9, correlating the difference in the amounts of said ibase used in the neutralizations to obtain an index of the free sulfuric acid content of said stream, and regulating, as required, the amount of said concentrated sulfur-ic acid being reacted'with said phosphatic material and recycled Weak phosphoric acid mass and the amount of the Wash Water, as required, in accordance with the determined free sulfuric acid content so as to maintain the desired amount of free sulfuric acid in the recycled weak phosphoric acid.

2. The process according Ito claim 1 wherein said slurry is continuously analyzed for said free sulfuric acid content.

3. The process according yto claim 1 wherein said re- 15 5. The process according to claim 1 wherein said free sulfuric acid content is maintained within the range of 2 and 2.5 percent.

References Cited in they file of this patent UNITED STATES PATENTS 1,940,689 Moore Dec. 26, 1933 2,710,247 Knowles June 7, 1955 2,887,362l Lee May 19, 1959 2,953,440

Claudy Sept. 20, 1960 

1. IN THE PROCESS FOR THE PRODUCTION OF PHOSPHORIC ACID ACCORDING TO THE WET PROCESS WHEREIN A FEED OF PHOSPHATIC MATERIAL AND SULFURIC ACID ARE CONTROLLED, SAID PROCESS INCLUDING THE STEPS OF INITIALLY REACTING PHOSPHATIC MATERIAL WITH RECYCLED WEAK PHOSPHORIC ACID, REACTING THE RESULTING MEANS WITH CONCENTRATED SULFURIC ACID TO OBTAIN A SLURRY OF PHOSPHORIC ACID AND GYPSUM, FILTERING SAID SLURRY IN A FILTRATION ZONE TO OBTAIN A STRONG ACID FILTRATE COMPRISING STRONG PHOSPHORIC ACID AND A REMAINING MASS COMPRISING GYPSUM, WASHING THE REMAINING GYPSUM MASS WITH A WASH MEDIUM TO OBTAIN A FIRST WEAL FILTRATE, WASHING THE NOW REMAINING MASS WITH WATER TO OBTAIN A SECOND WEAL ACID FILTRATE, USING SAID SECOND WEAK ACID FILTRATE AS SAID WASH MEDIUM, AND RECYCLING WEAK FILTRATE FOR USE AS SAID RECYCLED WEAK PHOSPHORIC ACID, THE IMPROVEMENT COMPRISING ANALYZING THE FREE SULFURIC ACID CONTANT IN A STREAM RESULTING IN THE PROCESS AND CONTAINING FREE SULFURIC ACID BY DILUTING A PORTION OF SAID STREAM WITH WATER, SUBSEQUENTLY NEUTRILIZING THE RESULTING DILUTED PORTION WITH STANDARD SODIUM HYDROXIDE BASE TO A PH OF ABOUT 4.8 AND THEN TO A PH OF ABOUT 9, CORRELATING THE DIFFERENCE IN THE AMOUNTS OF SAID BASE USED IN THE NEUTRALIZATIONS TO OBTAIN AN INDEX OF THE FREE SULFURIC ACID CONTENT OF SAID STREAM, AND REGULATING, AS REQUIRED, THE AMOUNT OF SAID CONCENTRATED SULFURIC ACID REACTED WITH SAID PHOSPHATIC MATERIAL AND RECYCLED WEAK PHOSPHORIC ACID MASS AND THE AMOUNT OF THE WASH WATER, AS REQUIRED, IN ACCORDANCE WITH THE DETERMINED FREE SULFURIC ACID CONTENT SO AS TO MAINTAIN THE DESIRED AMOUNT OF FREE SULFURIC ACID IN THE RECYCLED WEAK PHOSPHORIC ACID. 